This invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government or government purposes without payment of any royalties thereon or therefor.
1. Field of the Invention
This invention relates generally to the preparation and fabrication of prepreg tapes and ribbons and relates specifically to the preparation of polyimide resin/carbon fiber/boron fiber unidirectional solvent-free tape and ribbon having well wet-out and encapsulated boron fiber evenly distributed throughout the tape or ribbon, controlled dimensions and resin content, and composite structures having high mechanical properties.
2. Description of the Related Art
Reinforcing fibers comprising filaments combined with a matrix resin are known in the art and typically are called xe2x80x9ctowpregs.xe2x80x9d A conventional towpreg consists of thousands of filaments impregnated with a continuous mass of matrix. The type of advanced reinforcing fibers typically used are available commercially in bundles of filaments known as xe2x80x9ctows.xe2x80x9d The number of filaments vary widely per tow and is denoted by the tow count. Many matrix resins are available that generally fall into one of two resin types within the related art: thermoplastic and thermoset polymers.
Thermoplastic polymers have been used widely as matrices for composites, and are potentially useful as matrices for advanced composites in aerospace applications. Thermoplastics have advantages over thermosetting materials in fracture toughness, impact strength, and environmental resistance. Thermoplastics also provide towpregs with indefinite shelf life, give the fabricator better quality assurance, and avoid the storage and refrigeration problems associated with thermosetting towpreg. Thermoplastic molecules are tougher than the rigid crosslinked network of the thermosets; few of the toughened thermosets have met the combined requirements of damage tolerance and hot/wet compression strength necessary for use in aerospace composites. The disadvantage of thermoplastic polymers as a composite matrix material is the difficulty of uniformly coating the fibers due to the high viscosity of the molten polymer.
Thermoset polymers also are used as matrices for towpreg. Typically, towpreg containing thermosetting prepolymer, although relatively flexible, is tacky, thus requiring a protective release coating, which must be removed before use. While thermoset towpreg is acceptable for filament winding, its tackiness and the requirement of a protective release coating make thermoset towpreg unfeasible for weaving, braiding, or the production of any chopped fiber feed stock for bulk or sheet molding compounds.
Continuous fiber towpregs can be produced by a number of impregnation methods including hot melt, solution, emulsion, slurry, surface polymerization, fiber commingling, film interleaving, electroplating, and dry powder techniques. A powder impregnation method and apparatus are disclosed in U.S. patent application Ser. No. 09/185142, filed Nov. 3, 1998, entitled Method and Apparatus to Fabricate a Fully-consolidated Fiber-Reinforced Tape from Polymer Powder Preimpregnated Fiber Tow Bundles for Automated Tow Placement (Belvin et al.), now abandoned the disclosure of which is herein incorporated by reference. U.S. patent application Ser. No. 09/185142 discloses the manufacture of a 3-inch wide product from powder pre-impregnated fiber-tow bundles that employ a number of techniques that are very specific to the fabrication of a placeable-grade 3-inch wide product using powder prepreg fiber-tow bundles as the precursor.
Precursor fabrication is completed in various techniques, such as the Powder Curtain Process (PCP) and other slurry operations. In the PCP, the resin powder is mechanically deposited onto the fiber tow bundles while being pulled through a series of process components. The use of a powder for the impregnation of fiber tow bundles creates a number of obstacles the tape manufacturer has to overcome for the fabrication of a placement grade product.
The PCP can use a powder having a particle size ranging from 3 microns to 15 microns. The particle size is important in the tape fabrication process due to the melt viscosity of the resin. The melt resembles a droplet on the tow bundle and may not completely wet-out (encapsulate) the filaments in the tows. During periods when the resin does not wet-out (encapsulate) the filaments of the tows, the shape of the tape/ribbon becomes irregular and jagged which facilitates the generation of voids. The lack of a smooth uniform surface and a large void content inhibits the placement process during the fabrication of a component.
In any of the mentioned techniques (PCP, Slurry) of powder impregnation, the main concern of the tape manufacturer is the resin content along the length of the tows. If the resin content varies to a large extent, dry areas will exist through-out the tape, and/or an over abundance of resin will be localized in one area. With dry areas, voids manifest themselves during the fabrication of the tape and are magnified during the placement process, creating a product that does not perform as predicted in operation. If the resin content is high in a localized area, the mechanical properties become more dependent on the resin than is typical for that area. A highly consistent resin application and distribution along the length of the fiber tow bundles generates a well consolidated tape product, allowing the automated tape placement machine to fabricate a low void, well consolidated part.
The significant advantages of the solution-coating method include ensuring a virtually even distribution of a coating on the towpregs and the elimination of voids during the tape fabrication and tow placement processes. The ultimate goal for almost all solution-coating applications is the ability to deposit a thin, even thickness, high quality coating as efficiently as possible. The polymeric matrix or resin also must be soluble at ambient and refrigeration storage temperatures.
Typically, towpregs made from solution-coated fiber bundles are not universally well-characterized geometrically, leading to difficulties in using such towpregs for processes when an accurate geometry is vital for the production of high-quality parts. Examples of processes which require an accurate geometry include filament winding, pultrusion, and automated tow placement, or ATP.
ATP is a process where composite ribbons or tapes are robotically managed and continually fed onto a tool or part surface and adhered by application of heat and pressure. ATP is particularly sensitive to the quality of the ribbon when considering low-flow matrix materials. The simultaneous assembly of adjacent ribbons (typically 4 to 34) or wide tape offers significant advances in the lay-up of composite materials. However, ribbons or tapes made from low-flow matrix materials typically lack a cross-sectional dimensional integrity, and more importantly, a standard rectangular cross-section. These structural defects complicate the ATP process and frequently render poor results. Although ribbons are bonded to their vertical neighbor (directly below) satisfactorily, the failure to make quality parts is generally attributed to the poor bonding of adjacent ribbons to each other. Low-flow thermoplastic parts made by using slit prepreg tapes are typically unconsolidated and exhibit excessive porosity and void content.
Ideally, tapes used in the ATP process are fully consolidated. Consolidation can be defined as the elimination of voids in a composite material during melt-processing. One method of accomplishing consolidation is pultrusion. This technique requires full ingestion of the unconsolidated composite material within an enclosed die with an exit area less than the inlet area. Within the heated closed die, processing of the polymeric matrix forces the polymer melt to flow axially to the filament array, whereas flow transverse to the filament array is generally 1/10 to 1/100 of the axial flow. As a consequence of the geometry and boundary limits of the pultrusion die, voids must be expelled axially, against the flow of the composite material through the entrance of the pultrusion die. This complex flow of voids is known to limit the rates at which pultrusion may proceed. With the desirable prepreg attribute of low void content, the pultrusion process is limited in the length of the die because the longer the die, the longer the voids must travel to be fully expelled. This length contributes to a very slow production rate.
High performance polyimides are used in the aerospace industry, for example, in joining metals to metals, or metals to composite structures. In addition, polyimides are rapidly finding new uses as matrix resins for composites, molding powders and films. These materials display a number of performance characteristics such as high temperature and solvent resistance, improved flow for better wetting and bonding, high modulus, chemical and hot water resistance, and the like. Another area of application is in the manufacture of lighter and stronger aircraft and space structures.
U.S. Pat. No. 5,147,966 (St. Clair, et al.) discloses polyimides that can be melt processed into various useful forms as coating, adhesives, composite matrix resins and films. These polyimides are prepared from various dianilines and anhydrides in various solvents. The use of anhydrides as endcapping agents are also disclosed to control the molecular weight of the polymer and, in turn, to make it easier to process in molten form.
Current technology for making prepreg and composites from polyimides as described above utilized solutions from the poly(amide) acids of these resins. Poly(amide) acid solutions are processed into prepreg with various reinforcing fibers. These poly(amide) acid solutions are of low solids contents and high viscosity. In general, poly(amide) solutions are prepared at solid contents of 25 to 35% by weight with resulting Brookefield viscosities at 20xc2x0 C. of 15,000 to 35,000 cp. Therefore, the processing of these types of solutions requires overcoming significant problems such as solvent management and good fiber wet out from the high viscosity solutions. The resultant prepreg typically requires residual solvent contents of 20 to 25% by weight (approximately 2-3% water from thermal imidization reaction) for adequate tack and drape. This residual solvent must then be removed during the composite cure cycle. This material is hand-laid into composites which makes working with this type of material very labor intensive and costly.
Typically, carbon fiber composites can provide excellent mechanical properties. However, in certain applications carbon fiber alone does not provide adequate compression properties. A solution to this problem is the hybridization of carbon fiber composites with boron reinforcing fibers. Current technology for making boron/carbon prepreg and composites from polyimides utilizes solution coated prepreg with residual solvent. Boron fibers are calendered onto xe2x80x9cwetxe2x80x9d prepreg to make a hybrid boron/carbon fiber prepreg. This technique only pushes the large diameter boron fibers partially into the xe2x80x9cwetxe2x80x9d prepreg. The resultant prepreg contains boron fibers that are essentially sitting on the prepreg surface, not fully encapsulated with resin. This material also requires solvent removal during cure as well as being labor intensive.
The need to process high temperature polyimides into composites with minimal solvent is apparent. The hazards and expense of solvent removal and recovery are critical to this composite technology. Developing a dry hybrid polyimide tape allows for automated tape placement by a robot. This can significantly reduce the cost of processing these materials into composite parts and improve their compressive properties. A process which utilizes significantly less, or no, solvent and results in a higher quality intermediary and end product is key to the use of these polyimide systems in large quantities.
Accordingly, an object of the present invention is to manufacture a boron reinforced polymer matrix composite.
Another object is to manufacture a composite tape of essentially consistent thickness across its width.
A further object of the invention is to manufacture a composite tape suitable for automated tow placement.
Another object of the invention is to manufacture a fully-consolidated composite tape with minimal voids therein.
Still another object of the present invention is to manufacture a composite tape having geometric accuracy.
These objects are accomplished by the present invention that includes a method of manufacturing a hybrid boron reinforced polymer matrix composite comprising the steps of feeding solution-coated precursor tape through an alignment device to align the precursor tape laterally, wherein the solution-coated precursor tape comprise a resin content of from about 50 wt % or less; applying at least one linear array of boron fibers along the length of the solution-coated precursor tape; heating the precursor tape and applied boron fibers to a specific processing temperature which processes the polymeric matrix of the precursor tape; encapsulating the boron fibers between two layers of heated precursor tape forming a hybrid boron reinforced polymer matrix composition; cooling the hybrid boron reinforced polymer matrix composition to a temperature below the Tg of the hybrid boron reinforced polymer matrix composition forming a composite; and taking-up the hybrid boron reinforced polymer matrix composition composite.
The present invention further includes a hybrid boron reinforced polymer matrix composite produced from the process comprising the steps of providing solution-coated precursor tape, wherein the solution-coated precursor tape comprises a polyamic acid solution; applying at least one linear array of boron fibers along the length of the solution-coated precursor tape; heating the precursor tape and applied boron fibers to a specific processing temperature which processes the polymeric matrix of the precursor tape; and encapsulating a boron component into the solution NASA coated precursor tape, wherein a boron reinforced polymer matrix composition is formed.
Additionally, the present invention includes an apparatus for producing a hybrid boron reinforced polymer matrix composite, comprising a dispensing means for supplying at least one linear array of boron fibers; an applying means for positioning solution-coated precursor tape along a length of the dispensed linear array of the boron fibers; a processing component for heating the positioned precursor tape and linear array of the boron fibers to a specific processing temperature which processes the polymeric matrix of the precursor tape and encapsulates the boron fibers into the precursor tape, the processing component having an entrance and an exit, the processing component including an impregnation bar assembly positioned near the exit of the processing component for wetting-out and spreading the heated precursor tape with boron fibers to an initial width; a variable dimension forming nip means, positioned in operable relationship to the processing component, for shaping the heated precursor tape into a predetermined width, the variable dimension forming nip means having at least two rollers that are actively cooled, the rollers being forced together under a selected pressure, wherein the hybrid boron reinforced polymer matrix composition is formed; a driving means, positioned in operable relationship to the variable-dimension forming nip means, for pulling the shaped hybrid boron reinforced polymer matrix composition and maintaining a constant speed across the width of the hybrid boron reinforced polymer matrix composition, thereby enabling the polymeric matrix of the hybrid boron reinforced polymer matrix composition to consolidate fully into the boron reinforced polymer matrix composition; and a take-up component for taking-up the boron reinforced polymer matrix composition, wherein the final tape has less than about 2% voids. The method for manufacturing the composite tape can begin with the solution-coated precursor tape being mounted onto the pay-out creel for delivery. The precursor tape is tensioned at this point to facilitate alignment of the precursor tape within an apparatus. This tension also aids in the spreading of the precursor tape in the processing component. The precursor tape can then be fed through the aligning device to maintain the alignment of the precursor tape during processing. The aligning device facilitates the consistent thickness across the width of the processed material. If the alignment changes, a tape or ribbon will be fabricated of irregular shape unsuitable for use later with the ATP process.
In addition to the precursor tape, a linear array of boron fibers is dispensed and fed through the aligning device and applied along the length of the precursor tape. This alignment facilitates the forming of the molten pre-preg or polymeric matrix into a precise shape and dimension. The precursor tape and applied boron fibers then proceed through the processing component. The processing component can comprise two parts, an oven or furnace and an impregnation or stationary bar assembly. The oven is heated to a specific processing temperature for each individual polymeric matrix depending on the solution-coated resin of the precursor tape. The oven further removes most of the solvent on the precursor tape to an amount of from about 2 wt % or less, or more preferably from about 1 wt % or less. Preferably, when processing requires a high temperature to melt the polymeric matrix material, an inert gas such as nitrogen is used as a process medium inside the oven to induce melting without oxidation. While still inside the oven, the precursor tape and boron fibers can be pulled through the impregnation bar assembly. The bars facilitate the wetting out of the filaments of the precursor tape, encapsulates the boron fibers into the precursor tape, and aid in the initial spreading of the precursor tape to a selected width and shape. The tension created from the pay-out creel is instrumental in this spreading process, with greater tension further assisting the spreading of the precursor tape.
Upon exiting the process component, the molten precursor tape can be fed through the variable dimension forming nip means. The variable dimension forming nip means cools the molten precursor tape with boron fibers and shapes them into an essentially precise, predetermined width. Preferably, the invention uses nitrogen as the cooling medium. Additionally, because the variable dimension forming nip means preferably do not have a defined gap between the two rollers, the rollers allow for changes in resin content along the precursor tape during processing by varying the cross-section along the length of the composite tape. Resin content can vary along the length of the precursor tape as much as about xc2x18%. Generally, the resin content ranges from about 50 wt % or less, preferably from about 25 wt % to about 40 wt %, and more preferably from about 30 wt % to about 35 wt %.
The next component is the driving means. The driving means pulls the precursor tape and boron fibers to fabricate the hybrid boron reinforced polymer matrix composition through the process. The driving means can maintain the speed of the process and remove any speed differential within the precursor tape. This constant speed in turn eliminates a shearing force which would facilitate gaps and splits in the finalized tape. Thus, the driving means allows the resin content to flow together. As a result, the method produces an essentially fully-consolidated hybrid boron reinforced polymer matrix composite, which is spooled by a motorized take-up system.